1. Field of the Invention
The present invention relates to a method for producing a sintered cubic boron nitride compact having high hardness and uniformity. In particular, it relates to a production method allowing to obtain a sintered cubic boron nitride compact having high uniformity, high-density, and hardness, without using a binder, under a condition which is relatively low pressure and temperature (5 GPa or higher and 1400° C. or higher) comparing to conditions used in the past.
Priority is claimed on Japanese Patent Application No. 2010-026539, filed Feb. 9, 2010, the content of which is incorporated herein by reference.
2. Description of Related Art
Conventionally, for cutting of an iron-based workpiece, such as high hardness steel, a material with low compatibility to the iron-based workpiece, such as a sintered cubic boron nitride (hereinafter, referred to as cBN) compact, has been used.
In general, the sintered cBN compact is produced by mixing cBN starting powders with a binder, such as metal and ceramic, and by treating the mixture with ultra-high pressure and high temperature. However, because the sintered compact contains the binder, hardness and thermal conductivity of the sintered cBN compact deteriorate, and thereby failing to perform as good as it supposes to.
To solve the problem caused by using a binder, a method for producing a sintered cBN compact having high purity has been proposed. In this method, the cBN starting powders are cleansed by a supercritical fluid which is free of oxygen, and the cleansed cBN starting powders are sintered under a high-pressure and high-temperature condition without a binder, as disclosed in Japanese Unexamined Patent Application, First Publication No. 2007-70148. In this method, a source material of the supercritical fluid is one or more selected from a group consisting of polyvinylidene chloride, polyvinyl chloride, and polyethylene in solid state, and the sintering pressure is 5 GPa or higher and the sintering temperature is 1400° C. or higher. According to this producing method, it is reported that a highly pure sintered cBN compact that has high hardness and heat resistance can be obtained.
As an alternative technology not using a binder, a method for producing a sintered cBN compact has been proposed, where the cBN starting powders are sintered without a binder in an ultra-high pressure and high temperature condition in which cBN is thermodynamically stable. In this method, the sintering pressure is 7 GPa or higher, and the sintering temperature is 2100° C. or higher, as disclosed in Japanese Unexamined Patent Application, First Publication No. H3-159964. However, this method is not practical for higher production volume. Particularly, having a production apparatus for the sintered cBN compact of this method in larger scale, would cause not only higher cost but also higher difficulty to operate the apparatus. Thus, this method is not practical.